Effects of CO2/N2/Ar addition on liftoff of a laminar CH4/air diffusion flame
Résumé
Exhaust gas recirculation (EGR) is an effective method to lower peak flame temperatures and reduce pollutant emissions. It has been widely applied in industrial applications, such as high EGR diesel combustion and high temperature air combustion (flameless combustion). Introduction of exhaust gas into a combustion system may result in variations in some flame properties. One of these variations is that the addition of exhaust gas may cause the transition of a diffusion flame from attached to lifted, i.e. flame liftoff. Understanding the mechanism of the liftoff due to the introduction of exhaust gas is of great help for the further development and improvement of combustion technology employing EGR. Many studies have been devoted to attached or lifted diffusion flames. Generally the introduction of an additive to the oxidant or fuel stream of a diffusion flame causes variation in flame liftoff and other properties due to the change in combustion intensity. This change in combustion intensity is caused by five possible mechanisms. The first one is the dilution effect resulted from the decrease in the concentration of oxygen or fuel. The second one is the thermal effect caused by the change in specific heat. Thirdly, some components of exhaust gas may participate in chemical reactions and thus cause flame liftoff. This factor is referred to as chemical effect. In addition, some components of exhaust gas may alter flame temperature and cause liftoff by modifying radiation heat transfer rate. This effect is known as radiation effect. The last one is the transport property effect which is due to the difference in transport properties between the exhaust gas and fuel or oxidant. Carbon dioxide (CO2) and nitrogen (N2) are two primary components of exhaust gases. Most previous studies have attributed the impacts of CO2 and N2 addition on the extinction or liftoff of a diffusion flame to thermal and dilution effects [3]. While this is true for N2 addition, it may not be correct for CO2 addition. Our previous study [4,5] showed that the addition of CO2 affects some flame properties due to not only thermal and dilution effects, but also chemical effect. Besides, CO2 addition may also modify a flame through radiation effect. Few studies have been reported on the relative importance of the five possible mechanisms on the liftoff of a diffusion flame, when CO2 or N2 is added.